The cultivation of agricultural crop plants serves mainly for the production of foodstuffs for humans and animals. Monocultures in particular, which are the rule nowadays, are highly susceptible to an epidemic-like spreading of diseases. The result is markedly reduced yields. To date, the pathogenic organisms have been controlled mainly by using pesticides. Nowadays, the possibility of directly modifying the genetic disposition of a plant or pathogen is also open to man.
Resistance generally means the ability of a plant to prevent, or at least curtail the infestation and colonization by a harmful pathogen. Different mechanisms can be discerned in the naturally occurring resistance, with which the plants fend off colonization by phytopathogenic organisms. These specific interactions between the pathogen and the host determine the course of infection (Schopfer and Brennicke (1999) Pflanzenphysiologie, Springer Verlag, Berlin-Heidelberg, Germany).
With regard to the race specific resistance, also called host resistance, a differentiation is made between compatible and incompatible interactions. In the compatible interaction, an interaction occurs between a virulent pathogen and a susceptible plant. The pathogen survives, and may build up reproduction structures, while the host mostly dies off. An incompatible interaction occurs on the other hand when the pathogen infects the plant but is inhibited in its growth before or after weak development of symptoms. In the latter case, the plant is resistant to the respective pathogen (Schopfer and Brennicke, vide supra). However, this type of resistance is specific for a certain strain or pathogen.
In both compatible and incompatible interactions a defensive and specific reaction of the host to the pathogen occurs. In nature, however, this resistance is often overcome because of the rapid evolutionary development of new virulent races of the pathogens (Neu et al. (2003) American Cytopathol. Society, MPMI 16 No. 7: 626-633).
Most pathogens are plant-species specific. This means that a pathogen can induce a disease in a certain plant species, but not in other plant species (Heath (2002) Can. J. Plant Pathol. 24: 259-264). The resistance against a pathogen in certain plant species is called non-host resistance. The non-host resistance offers strong, broad, and permanent protection from phytopathogens. Genes providing non-host resistance provide the opportunity of a strong, broad and permanent protection against certain diseases in non-host plants. In particular such a resistance works for different strains of the pathogen.
Fungi are distributed worldwide. Approximately 100 000 different fungal species are known to date. The rusts are of great importance. They can have a complicated development cycle with up to five different spore stages (spermatium, aecidiospore, uredospore, teleutospore and basidiospore).
During the infection of plants by pathogenic fungi, different phases are usually observed. The first phases of the interaction between phytopathogenic fungi and their potential host plants are decisive for the colonization of the plant by the fungus. During the first stage of the infection, the spores become attached to the surface of the plants, germinate, and the fungus penetrates the plant. Fungi may penetrate the plant via existing ports such as stomata, lenticels, hydatodes and wounds, or else they penetrate the plant epidermis directly as the result of the mechanical force and with the aid of cell-wall-digesting enzymes. Specific infection structures are developed for penetration of the plant. The soybean rust Phakopsora pachyrhizi directly penetrates the plant epidermis. After crossing the epidermal cell, the fungus reaches the intercellular space of the mesophyll, where the fungus starts to spread through the leaves. To acquire nutrients the fungus penetrates mesophyll cells and develops haustoria inside the mesophyl cell. During the penetration process the plasma membrane of the penetrated mesophyll cell stays intact. Therefore the soybean rust fungus establishes a biotrophic interaction with soybean.
Immediately after the attachment of the spore to the plant cell, the plants sense the presence of the pathogen. Mostly the presence of the pathogen is sensed via so called PAMP receptors, a class of trans-membrane receptor like kinases recognizing conserved pathogen associated molecules (e.g. flagellin or chitin). Downstream of the PAMP receptors, a signaling cascade is initiated leading to the elicitation of defense reactions. On the other hand the pathogen is counteracting the defense related signaling pathway by injecting so-called effector proteins into the host cell. The effectors lead to the inhibition, weakening or deregulation of the defense regulation, in a way that the pathogen can further colonize the plant. But these effectors might also be recognized by the plant by major R- (resistance) genes of the NBS-LRR class, leading to a strong defense response.
During million years of evolution, every pathogen shaped its effector set in a way to block exactly those defense reactions that are most effective against the particular pathogen. So, the most interesting genes, in terms of creating a disease resistant plant, are those genes that are suppressed by the pathogen in their respective host plant.
Immediately after recognition of a potential pathogen the plant starts to elicit defense reactions.
Soybean rust has become increasingly important in recent times. The disease may be caused by the biotrophic rusts Phakopsora pachyrhizi (Sydow) and Phakopsora meibomiae (Arthur). They belong to the class Basidiomycota, order Uredinales, family Phakopsoraceae. Both rusts infect a wide spectrum of leguminosic host plants. P. pachyrhizi, also referred to as Asian rust, is an aggressive pathogen on soy (Glycine max, Soja hispida or Soja max), and is therefore, at least currently, of great importance for agriculture. P. pachyrhizi can be found in nearly all tropical and subtropical soy growing regions of the world. P. pachyrhizi is capable of infecting 31 species from 17 families of the Leguminosae under natural conditions and is capable of growing on further 60 species under controlled conditions (Sinclair et al. (eds.), Proceedings of the rust workshop (1995), National SoyaResearch Laboratory, Publication No. 1 (1996); Rytter J. L. et al., Plant Dis. 87, 818 (1984)). P. meibomiae has been found in the Caribbean Basin and in Puerto Rico, and has not caused substantial damage as yet.
P. pachyrhizi can currently be controlled in the field only by means of fungicides. Soy plants with resistance to the entire spectrum of the isolates are not available. When searching for resistant plants, four dominant genes Rpp1-4, which mediate resistance of soy to P. pachyrhizi, were discovered. The resistance was lost rapidly, as P. pychyrhizi develops new virulent races.
In recent years, fungal diseases, e.g. soybean rust, has gained in importance as pest in agricultural production. There was therefore a demand in the prior art for developing methods to control fungi and to provide fungal resistant plants.
Much research has been performed on the field of powdery and downy mildew infecting the epidermal layer of plants. However, the problem to cope with soybean rust which infects the mesophyll remains unsolved. This problem is worsened by the fact that prior to the present invention no prediction could be made regarding plant genes involved in resistance against soybean rust. Thus, constructing resistant plants by means of genetic modification and/or breeding was a matter of try and error, severely reducing the chances of obtaining resistant plants. Also, as no prediction could be made regarding plant genes involved in resistance against soybean rust, it was also not possible to screen in any technically meaningful way for chemical agents increasing expression of genes involved in soybean rust resistance.